1. Field of the Invention
The present invention relates to an image forming apparatus for depositing toner, or developer, on a latent image formed on a photoconductive drum or similar image carrier to thereby form a corresponding toner image and transferring the toner image to a paper sheet or similar recording medium.
2. Description of the Background Art
An image forming apparatus includes a developing device for developing a latent image formed on an image carrier. It is a common practice with a developing device to deposit toner agitated in the apparatus on a developing roller or developer carrier. The developing roller is rotated to convey the toner to a position where the roller faces the image carrier, so that the toner is transferred to a latent image formed on the image carrier to thereby develop the latent image. The toner left on the developing roller after development is collected in the apparatus due to the rotation of the developing roller. Fresh toner is charged by agitation and again deposited on the developing roller.
Japanese Patent Laid-Open Publication No. 5-19615 discloses another type of developing device in which toner is electrostically conveyed on the surface of a developing roller and then transferred to the surface of an image carrier by attraction, which acts between the developing roller and the image carrier. Japanese Patent Laid-Open Publication No. 59-181375, for example, proposes a developing device including a conveyance board for electrostatically conveying toner to a position where the board faces an image carrier. The toner is then separated from the conveying surface of the conveyance board by attraction acting between the board and an image carrier and transferred to the image carrier.
Further, Japanese Patent Laid-Open Publication Nos. 11-170591, 11-115235 and 11-179951, for example, each teach an image forming apparatus of the type causing toner to fly from a developing roller to a recording medium. This type of apparatus includes control electrodes arranged between a developing roller and a recording medium and counter electrodes located at the rear of the recording medium. Electric fields are generated between the developing roller and the counter electrodes, so that toner can fly toward the recording medium. The control electrodes selectively control the flight of the toner to thereby form an image on the recording medium.
The image forming apparatus of the type transferring toner from the developing roller to the image carrier or causing toner to fly by controlling the electric fields has the following problem. The developing roller essential with such a type of image forming apparatus increases the overall size and cost of the apparatus. Further, the problem with the developing device using the developing roller is that toner enters a gap between the roller and side walls and coheres due to friction, degrading image quality. With the developing device of the type electrostatically conveying toner, it is impracticable to surely convey the toner.
When toner is charged by friction or corona discharge, toner particles reached saturation charge and toner particles not reached it exist together, resulting in a broad charge distribution. Assume that such toner is forcibly transferred to an image carrier by, e.g., a magnet brush or a transfer roller. Then, the toner particles with low charge and deposited on the image carrier are apt to leave the it due to the developing speed of the state-of-the-art developing roller, i.e., about 100 cm/sec in terms of linear velocity. The toner particles left the image carrier fly about or deposit on the background of an image.
Moreover, the particle size of toner for development or image formation should preferably be uniform as far as possible. The conventional image forming apparatuses or developing devices thereof feed toner particles sized less than 5 xcexcm to a developing roller together with the other toner particles. Let toner particles, or powder, sized less than 5 m be referred to as extremely fine toner particles hereinafter.
It is an object of the present invention to provide an image forming apparatus and a developing device that are simple in construction, low cost, and high in image quality.
It is another object of the present invention to provide a powder jetting device feasible for the developing device.
It is still another object of the present invention to provide a toner feeding device capable of uniforming the particle size of toner to be fed to a developing device to thereby enhance image quality.
It is yet another object of the present invention to provide a toner conveying device for conveying toner to an image carrier or a recording medium.
It is a further object of the present invention to provide a classifying device for classifying powder.
In accordance with the present invention, an image forming apparatus includes an image carrier for forming a latent image thereon, and a developing device for developing the latent image with charged toner to thereby form a corresponding toner image. The developing device electrostatically conveys the charged toner toward one end of a conveyance board along the conveying surface of the board and causes it to fly toward the image carrier from the one end.
Also, in accordance with the present invention, in a toner conveying device for electrostatically conveying toner, a conveyance board includes a first and a second board stacked on each other. The first board includes a substrate and a plurality of substantially parallel electrodes arranged at a preselected distance in the direction of toner conveyance and each extending in the direction crossing the above direction. The second board includes an insulative substrate and a surface layer formed on the surface of the insulative substrate and having low contact resistance with respect to the toner. The surface layer forms a conveying surface.